Technologies like microelectronics, micromechanics and biotechnology have created a high demand in industry for structuring and probing specimens within the nanometer scale. On such a small scale, probing or structuring is often done with electron beams, which are generated and focussed in charged particle beam devices like electron microscopes or electron beam pattern generators. Charged Particle beams offer superior spatial resolution compared to e.g. photon beams due to their short wavelengths.
The spatial resolutions that could possibly be achieved based on wavelength e.g. less than 0.01 nm is, however, limited due to intrinsic aberrations and misalignments of the beam of charged particles leading to a decrease of resolution.
For example, in a scanning electron microscope (SEM) the beam is focused to a small spot having a size around 10 nm or smaller. The beam is scanned over a specimen. Thereby, the resolution of the images obtained is limited by the beam diameter in the plane of the sample surface.
The beam diameter can be limited by aberrations, for example chromatic aberrations that are independent of the alignment of the beam but depend on the energy variance of the electron beam. Further, there are spherical aberrations that are produced by non-zero apertures of the imaging lenses. However, aberrations can be made worse or can even be introduced by a misalignment of the beam with respect to the optical axis of the individual imaging element. Thus, one of the most important parameters for optimizing the resolution and, thereby, the imaging quality is the alignment of the optical column. Since the high spatial resolutions require very small tolerances, alignment of the beam with respect to individual optical elements has to be conducted on a regular basis.
Conventionally an alignment of a charged particle column has to be performed by an operator. Thereby, an operator adjusts the respective signals applied to an alignment correction devices based on images measured. One disadvantage of this procedure is that it is dependent on the judgment of the operator. Thus, inaccuracies and variations from one operator to the other are introduced. Further, the manually adjustments are time consuming which is particularly disadvantageous for online inspection systems requiring a high system throughput.
In document U.S. Pat. No. 5,627,373 a method for automatically aligning an electron beam axis to an objective lens axis in a scanning electron microscope is described. Thereby, an image of a specimen at a first and at a second point of focal range of the objective lens is measured. For each image an indication signal of a position of a straight edge within the field of view of the microscope is generated. After an image translation is detected from the two signals and alignment is automatically adjusted, the procedure is repeated in an orthogonal direction. The complete operating sequence is repeated in iterations until the image shift occurring due to the misalignment is less than a predetermined threshold.
Further, a method for automatically correcting electron beam astigmatism in a scanning electron microscope is suggested in document U.S. Pat. No. 5,627,373. For astigmatism correction, border portions are sampled about the entire specimen circumference at a 30° interval. An axis of beam distortion is identified upon the indication signals of sharpness among the samples. The distortion is adjusted along such axis and improved in iterations.
Document U.S. Pat. No. 6,025,600 teaches a method for calculating and correcting an astigmatism error in charged particle beam systems. Images are collected during a single sweep of the objective lens settings of the charged particle system. Different orientations of image features, such as lines in a stigmation target, are analyzed. Optimum sharpness or best focus values are obtained as a function of the objective lens settings. Appropriate changes to the settings of the astigmatism correctors are computed by taking a linear combination of optimum sharpness values associated with the different orientations of image features.
In document U.S. Pat. No. 6,067,167 a scheme for realizing the automatic adjustment of the electron optics system in an electron optics device such as scanning electron microscope is described in which a prescribed number of images sequentially obtained by the electron optics device at sequentially adjusted focus points due to a changing refractive index of an objective lens are stored. A moving amount of a sample image is calculated. Thereby, a judgment of whether an adjustment is necessary or not is based on the calculated moving amount and an adjustment is conducted if needed. Further, a scheme for realizing the astigmatism correction in a charged particle beam optical system of an electron optics device such as scanning electron microscope is disclosed.
However, there are further problems related to the alignment of a charged particle beam column and the improvements presented within the prior art needs further perfectioning especially when online inspection systems or online beam writers are considered.